The current process for forming a gas turbine engine casing is illustrated in FIGS. 1A, 1B, 1C and 1D. FIG. 1A illustrates a forging 10 created using a ring rolling process. The forging 10 has a cylindrical shape, where the axis of the cylinder extends into the page. The cross-section of the forging 10 illustrated in FIG. 1A is annular. The inner surface 12 of the forging 10 and the outer surface 14 of the forging 10 are concentric cylinders.
The forging 10 is used to form an axial split casing 20 for a gas turbine engine. It may be formed from corrosive resistant steel, titanium or nickel alloy.
FIG. 1B illustrates a rough machining which is carried out on the forging 10 of FIG. 1A before the casing 10 is split to form the casing 20. If an orthogonal coordinate system (X, Y) is defined with the origin on the axis of the cylindrical forging 10, then the forging is rough machined in the following way:
a. A lathe is used to remove material from the interior of the forging 10. The lathe cuts a cylindrical tube of radius R1 centred at (−X1, 0).
b. A lathe is used to remove material from the interior of the forging 10. The lathe cuts a cylindrical tube of radius R1 centred at (X1, 0).
c. A lathe is used to remove material from the exterior of the forging 10. The lathe cuts a cylindrical tube of radius R2 centred at (0, Y2).
d. A lathe is used to remove material from the exterior of the forging 10. The lathe cuts a cylindrical tube of radius R2 centred at (0,−Y2).
In FIG. 1B, the circular dashed lines indicate the original boundaries of the forging 10 before rough machining. The solid lines indicate the boundaries of the forging after rough machining. The dotted lines indicate lines at Y=−X1 and Y=+X1.
The forging is then axially split by removing the material between −X1 and +X1 as shown in FIG. 1C to form first 20a and second 20b portions of the axial split casing 20. Fine machining of the portions 20a, 20b is then carried out for example, by milling the interior and exterior of the portions.
The two portions 20a, 20b are then joined as shown in FIG. 1D, to form the axial split casing 20. It will be appreciated, that the axial split casing has a substantially cylindrical shape.
The rough machining of the interior and exterior of the forging illustrated in FIG. 1B is required to compensate for the removal of material between −X1 and +X1 when the forging is split axially so that the finished product, the axial split casing 20 is substantially cylindrical.
There are several problems associated with the above mentioned process. A considerable amount of material may need to be removed from the interior and the exterior of the forging during the rough machining process. This is a waste of material and also results in excessive wear to the lathes used to perform the rough machining.
It would therefore be desirable to provide an improved process for forming an axial split casing.